Location Sensing for Analytical Applications

ABSTRACT

Embodiments herein describe RFID systems that include multiple RFID tag readers that each use a different frequency to communicate with an RFID tag. For example, each of the tag readers may transmit a tag query command using different modulated frequencies. In one embodiment, the RFID tag includes multiple receivers each tuned to one of the different frequencies generated by the tag readers. For example, one receiver in the tag is tuned to receive  200  MHz signals while another receiver is tuned to receive  900  MHz signals. To provide location information, the RFID tag compares power values associated with the received signals to determine which of the RFID tag readers is closest to the tag. The RFID tag conveys this location information to the tag readers by selecting one of the frequencies of the tag readers to use when generating a reply message.

BACKGROUND

The present invention relates to radio frequency identification (RFID)tags, and more specifically, to generating location information usingmultiple transmission frequencies.

Typical RFID tags include an integrated circuit (IC) functionallyconnected to an antenna. The IC stores unique data for identifying aspecific item or user associated with the RFID tag. The IC alsomodulates a radio frequency (RF) signal that is transmitted orbackscattered via the antenna. An external tag reader captures the datasignal transmitted by the RFID tag.

RFID tags can be classified as “active” or “passive” devices. Activetags use an internal power source to actively transmit a modulatedsignal to the tag reader. Passive tags, in contrast, do not activelytransmit modulated signals to the tag reader but modulate theelectromagnetic waves scattered from the tag's antenna. The signal thatis scattered originates from the tag reader. Passive RFID tags arepowered by the incident signal transmitted by the tag reader. Passivetags are typically less expensive than active tags but have shorterfunctional ranges than active RFID tags. RFID tags typically operate inthe ultra-high frequency (UHF) or microwave frequency bands.

In some cases, the external tag reader may determine the distancebetween the reader and the RFID tag. Conventional ranging techniques,such as received signal strength indication (RSSI) for either active orpassive RFID tags, work over relatively large areas but suffer from pooraccuracy. Time of flight or frequency modulated continuous wave radarmay also be used to determine the separation distance but thesetechniques work poorly for short distance applications because of thedifficulties of measuring the small round trip time or frequency delay.

SUMMARY

One embodiment of the present invention is a method of operating a RFIDtag. The method includes receiving a first RF identification (RFID)signal at the RFID tag at a first frequency and receiving a second RFIDsignal at the RFID tag at a second frequency different from the firstfrequency. The method includes generating respective power valuesindicating the signal strengths of the first and second RFID signals anddetermining, based on the respective power values, which of the firstand second RFID signals has the greater signal strength. The methodincludes transmitting a reply message from the RFID tag using afrequency corresponding to the first or second RFID signal with thegreater signal strength.

Another embodiment of the present invention is an RFID tag that includesa first receiver tuned to receive a first RFID signal where the firstreceiver is configured to generate a first power value indicating asignal strength of the first RFID signal and a second receiver tuned toreceive a second RFID signal where the second receiver is configured togenerate a second power value indicating a signal strength of the secondRFID signal. Moreover, the first and second receivers are tuned todifferent frequencies. The RFID tag includes a comparator configured todetermine which of the first and second RFID signals has a greatersignal strength based on the first and second power values and atransmitter configured to transmit a reply message using a frequencycorresponding to the first or second RFID signal with the greater signalstrength.

Another embodiment of the present invention is an RFID system thatincludes a first tag reader configured to transmit a first RFID signalusing a first frequency and a second tag reader configured to transmit asecond RFID signal using a second frequency different from the firstfrequency. The RFID system includes a RFID tag that includes at leastone receiver configured to receive the first RFID signal and the secondRFID signal. The RFID tag also includes logic configured to select, inresponse to receiving the first and second RFID signals, one of thefirst and second frequencies by determining which of the first or secondRFID signal has the greater signal strength and output a reply messagecomprising a tag ID using the selected frequency.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an RFID system that uses multiple frequencies tocommunicate with an RFID tag, according to one embodiment disclosedherein.

FIG. 2 illustrates an RFID system that uses multiple frequencies tocommunicate with an RFID tag, according to one embodiment disclosedherein.

FIG. 3 is a flow chart for generating location information by comparingpower values corresponding to multiple frequencies, according to oneembodiment disclosed herein.

FIGS. 4A and 4B illustrate an RFID system for determining a location ofan RFID tag, according to one embodiment disclosed herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Embodiments herein describe an RFID system that includes multiple RFIDtag readers that each use a different frequency to communicate with anRFID tag. For example, each of the tag readers may transmit a tag querycommand using different modulated frequencies. In one embodiment, theRFID tag includes multiple receivers that are each tuned to receive oneof the different frequencies generated by the tag readers. For example,one receiver in the tag is tuned to receive 200 MHz signals whileanother receiver is tuned to receive 900 MHz signals. Moreover, thereceivers in the RFID tag may include respective power sources thatgenerate power from the signals received from the tag readers. In oneembodiment, the power sources generate a power value that corresponds toeach of the received frequencies—e.g., 200 and 900 MHz. These powervalues are used to determine a location of the RFID tag relative to thetag readers.

To generate location information, the RFID tag compares the power valuesgenerated by the power sources to determine which of the RFID tagreaders is closest to the tag. For example, if the power source on thetag that tuned to 200 MHz signals generates more power than the powersource tuned to 900 MHz signals, the RFID tag determines it is closer tothe RFID tag reader outputting the 200 MHz signal than the tag readeroutputting the 900 MHz. In response, the RFID tag can generate a replymessage at or near 200 MHz which indicates to the tag readers that theRFID tag is closest to the reader outputting the 200 MHz signal. Thatis, to convey location information, the RFID tag outputs a reply messageusing the frequency that generated the highest power value in the tag.By detecting the reply message, the tag reader can determine whether theRFID tag is within its region or zone.

FIG. 1 illustrates an RFID system 100 that uses multiple frequencies tocommunicate with an RFID tag 115, according to one embodiment disclosedherein. The RFID system 100 includes two RFID tag readers 105A and 105Bthat each includes respective transmitters 110. In one example, the tagreaders 105 transmit messages using different frequencies. For example,the transmitters 110 in each tag reader 105 may transmit tag querycommands using different frequencies to determine whether the RFID tag115 is in range. Although the embodiments herein recite that the tagreaders 105 use different frequencies (e.g., 200 and 900 MHz), thereaders 105 may use different (non-overlapping) frequency ranges. Forexample, transmitter 110A may use the frequency range 190-210 MHz togenerate a modulated data signal—e.g., a tag query command—to the RFIDtag 115, while transmitter 110B uses the frequency range 890-910 MHz togenerate modulated data signals.

The RFID tag 115 includes multiple receivers 120, a transmitter 125, andcontrol logic 130. In this example, the tag 115 includes two receivers120 that correspond to the two tag readers 105. The receivers 120 areeach tuned to one of the frequencies outputted by the tag readers 105.For example, receiver 120A may be designed to receive 200 MHz signalsgenerated by tag reader 105A, while receiver 120B is designed to receive900 MHz signals generated by tag reader 105B. However, the RFID system100 may include more than two tag readers 105 which output signals atunique frequencies which means the RFID tag 115 may include more thantwo receivers 120.

In another embodiment, the RFID tag 115 may include a single receiverfor receiving the different frequency signals transmitted by the tagreaders 105. Put differently, rather than having two distinct receiversthat are tuned to the different frequencies of the tag readers 105, theRFID tag 115 includes one receiver for receiving all the frequenciesemitted by the tag readers 105. For example, the receiver may include aninternal phase locked loop (PLL) that locks to whatever tag readerfrequency is the strongest and provides that information to the controllogic 130. Thus, the circuit elements in the PLL compares power valuescorresponding to the received tag reader signals and selects which ofthe signals has the greater signal strength.

In one embodiment, the control logic 130 compares power values generatedby the receivers 120 to determine which frequency corresponds to thegreater power value. As will be discussed in greater detail below, thereceivers 120 may include AC-DC converters, voltage or current supplies,charge pumps, and the like which generate power values using the signalsfrom the received from the transmitters 110 in the tag readers 105. Forexample, the receivers 120 may take the AC modulated RF signals andconvert these signals into power values such as a DC voltage or current.The control logic 130 determines the receiver 120 that outputs thegreatest power value thereby indicating which tag reader 105 is closerto the RFID tag 115.

Once the closest reader is identified, the control logic 130 instructsthe transmitter 125 to transmit a reply message using the frequencycorresponding to the closest tag reader 105. For example, if tag reader105B (which transmits a 900 MHz signals) is the closest, than thetransmitter 125 outputs the reply message using a 900 MHz modulatedsignal. Put differently, the control logic 130 can selectively controlthe transmitter 125 to output a reply message on any of the frequenciesused by the tag readers 105 in the RFID system 100. Thus, when the tagreader 105B receives the reply message generated using a 900 MHz signal,the reader 105B knows the tag 115 is closer to it than to the tag reader105A. In this manner, the tag 115 can transmit location information tothe readers 105.

One advantage of selectively controlling the transmitter is that doingso may avoid problems associated with multi-path reflections. Forexample, when the tag 115 transmits (or backscatters) the reply message,this message may reflect off different surfaces in the environment whichcauses multi-path signals to reach the readers 105. The tag readers 105may receive multiple delayed copies of the same reply message because ofthese multi-path signals. These delayed copies make determining thelocation of the RFID tag 115 difficult. However, because the embodimentsherein selectively control the frequency at which the tag 115 modulatesthe received signals, even if a tag reader 105 receives multiple delayedcopies of the reply message, the reader 105 can determine if it is theclosest reader to the tag 115 by determining if the modulation frequencyused by the tag 115 to generate the reply message is the same as itstransmission frequency used to output, for example, tag query commands.

If the tag 115 is moved such that it is now closer to tag reader 105Athan tag reader 105B, the control logic 130 in the tag 115 can detectthis change in location by identifying that the power value generated bythe receiver 120 tuned to the output frequency of tag reader 105A is nowgreater than the power generated by the receiver 120 tuned to the outputfrequency of tag reader 105B. In response, the control logic 130instructs the transmitter 125 to stop using the 900 MHz signal togenerate the reply messages and instead send messages using thefrequency corresponding to tag reader 105A—e.g., 200 MHz. In thismanner, the tag 115 provides updated location information as itslocation relative to the tag readers 105 changes.

FIG. 2 illustrates an RFID system 200 that uses multiple frequencies tocommunicate with the RFID tag 115, according to one embodiment disclosedherein. The RFID system includes RFID tag 115 and RFID tag reader 105.The tag reader 105 includes a transmitter 110 and receiver 230. In oneembodiment, the transmitter 110 transmits messages to the RFID tag 115using only one frequency. In one embodiment, the receiver 230 canreceive reply messages from the RFID tag 115 only on the sametransmission frequency used by the transmitter 110. For example, thetransmitter 110 may transmit query commands only at 200 MHz while thereceiver 230 can only receive messages at that same frequency—i.e., 200MHz. Having a receiver 230 that can only receive data at the samefrequency as the transmission frequency may reduce the cost of the tagreader 105.

Alternatively, the tag reader 105 may have a receiver 230 that canreceive messages at multiple frequencies, even if those frequencies areoutside the bandwidth of the transmitter 110—e.g., the receiver 230 maybe able to receive data messages sent by the RFID tag 115 at 200 MHz and900 MHz. Advantages for including a receiver 230 that can receivemessages at frequencies other than the transmission frequency used bythe transmitter 110 are discussed later.

The RFID tag 115 includes at least one antenna 205, multiple receivers120, the transmitter 125, a comparator 225, and the control logic 130.In one embodiment, the receivers 120 share the same wideband antenna 205for receiving data messages sent by tag readers 105 at differentfrequencies. Alternatively, each receiver 120 may be coupled to arespective antenna designed to receive signals at the same frequenciesat which the receivers 120 are tuned. Moreover, the transmitter 125 mayuse the same antenna 205 (or antennas) used by the receivers 120 or thetag 115 may include a separate transmitter antenna for transmittingreply messages to the RFID tag readers 105.

In this example, the RFID tag 115 is a passive tag that uses powercaptured from the RF signals transmitted by the tag readers 105 to powerits internal components (e.g., the transmitter 125, comparator 225,control logic 130, etc.) to generate and transmit reply messages to thetag readers 105. However, in other embodiments, the tag 115 may be anactive or semi-passive tag that includes an independent powersource—e.g., a battery. Even if the power source is used to transmit thereply messages, the comparator 225 can still compare a power value(e.g., signal strength) measured by each of the receivers 120 todetermine which tag reader 105 is closest to the RFID tag 115. Thecontrol logic 130 can then instruct the transmitter 125 to generate areply message using the frequency corresponding to the closest tagreader 105 as discussed above.

Because the tag 115 in FIG. 2 is passive, the receivers 120 eachincludes respective power supplies 210 which generate power values fromthe signals received from the tag readers 105. For example, the powersupplies 210 may include AC-DC converters such as charge pumps (e.g., aDickson charge pumps) which convert the AC signals into a DC voltageand/or current. The power values generated by the power supplies 210 maythen be used to power the transmitter 125, comparators 225 and controllogic 130. In one embodiment, the power supplies 210 may share the samecircuit design although the individual electrical values or sizes of thecircuit elements in the supplies 210 may vary in order to tune the powersupplies 210 to different frequencies.

The transmitter 125 includes a modulator 215 which modulates thereceived signal to generate a reply message. In one embodiment, thetransmitter 125 and modulator 215 use a backscatter technique to reflectand modulate the incident signals generated by the tag readers 105 totransmit data back to the tag readers 105. As shown, the modulator 215includes an ID 220 that may be stored in a non-volatile memory elementin the tag 115. When replying to a tag query command, the modulator 215modulates the received signal to include the ID 220. Thus, when themodulated signal reflected or backscattered by the RFID tag 115 reachesthe tag reader 105, the receiver 230 demodulates the signal andidentifies the ID 220. In this manner, the tag reader 105 can identifythe tag 115. In one embodiment, the ID 220 may be a product ID fortracking a product to which the tag 115 is attached, a user IDcorresponding to a badge that includes the tag 115, a vehicle ID inwhich the tag 115 is placed, and the like. In this manner, the tagreader 105 can transmit tag query commands which are then used by thetag 115 to generate modulated reply messages that include the ID 220.

In addition to providing the ID 220, the tag 115 may also transmitdifferent information in response to other commands transmitted by thetag reader 105. For example, the tag 115 may transmit security or accesscodes, a product expiration date (for perishable foods), timestamps, andthe like in response to commands transmitted by the tag reader 105.

The comparator 225 receives the power values generated by the receivers120 and determines which value is the greatest. In one embodiment, thepower values are a voltage or current that is generated by the powersupplies 210. Generally, the greater the signal strength of the signalreceived by the RFID tag 115, the greater the power values generated bythe power supplies 210. As such, although voltage or current arespecifically mentioned, the power values may be any indicator of thesignal strength of the signals transmitted by the tag readers 105 to theRFID tag 115. By comparing the power values, the comparator 225generates an output indicating which of the tag readers 105 is closer tothe RFID tag 115. The comparator 225 may include hardware elements suchas CMOS circuitry for determining which power value is the greatest. Inother embodiments, the comparator 225 may include a firmware or softwareoperation executed in an IC on the tag 115.

Using the output of the comparator 225, the control logic 130 instructsthe modulator 215 to generate the reply message using the frequency thatcorresponds to the closest tag reader 105. That is, if the tag 115receives signals at two different frequencies, the control logic 130instructs the modulator 215 to modulate the signal to include the ID 220at the frequency of the tag reader 105 that is closest to the tag 115.For example, if the tag 115 receives signals at 200 and 900 MHz but the200 MHz signal corresponds to a greater power value, the modulator 215modulates the 200 MHz signal to include the ID 220. In one embodiment,the control logic 130 instructs the modulator 215 to modulate only the200 MHz signal but not any of the other received signals (e.g., the 900MHz signal). Because the tag reader 105 generating the 200 MHz receivesthe valid ID 220, this reader 105 knows it is the closest reader 105 tothe tag 115.

In another embodiment, the logic 130 may instruct the modulator 215 tomodulate the 200 MHz signal to include the ID 220 and modulate the 900MHz signal to include different data informing the tag reader 105transmitting the 900 MHz that is not the closest tag reader. Forexample, the data sent to this tag reader 105 may be a predefined codeindicating to the reader 105 it is not the closest reader 105. In oneembodiment, the predefined code may include the ID 220 as well also asadditional data indicating to the reader 105 transmitting at 900 MHzthat another reader 105 is closer to the tag 115. By receiving thiscode, the tag reader 105 can still identify the tag 115 (and theassociated user or product) using the ID 220, but also determine thatanother tag reader 105 is closer to the tag 115.

In one embodiment, the receivers 120, transmitter 125, comparator 225and control logic may be embodied in an IC on the tag 115. For example,the tag 115 may include an integrated circuit that includes thecircuitry for performing the functions recited above. Moreover, the tag115 may include a combination of digital and analog circuitry.

FIG. 3 is a flow chart of a method 300 for generating locationinformation by comparing power values corresponding to multiplefrequencies, according to one embodiment disclosed herein. At block 305,the RFID tag receives tag query commands (e.g., RFID signals) frommultiple RFID tag readers using different frequencies. In oneembodiment, the RF system is configured such that each RFID tag readertransmits the tag query commands using a unique frequency ornon-overlapping range of frequencies. As such, each frequency can becorrelated to exactly one tag reader. As above, the RFID tag may includemultiple receiver circuits that are tuned to the different frequenciestransmitted by the tag readers.

At block 310, the RFID tag generates power values from the querycommands using differently tuned power supplies in the receivers. In oneembodiment, each of the receivers generates a power value (i.e., anindicator of signal strength) corresponding to one of the frequencies.For example, the receiver may include a charge pump that uses thesignals received from the tag readers to generate a voltage or currentfor powering the internal components in the RFID tag—i.e., to activatethe tag. However, if the RFID tag is an active or semi-passive tag, thepower value generated by the receivers may not be used to power internalcomponents. Rather, a separate power source in the RFID tag may be usedto power the tag and transmit reply messages to the tag readers.

At block 315, control logic in the RFID tag determines which frequencycorresponds to the greatest generated power. In one embodiment, thecontrol logic uses the power values (which are directly or indirectlycorrelated to the received power) to select one of the frequenciestransmitted by the tag readers. The control logic determines that thetag reader emitting the frequency corresponding to the highest powervalue is the closest tag reader.

At block 320, the control logic instructs the modulator to transmit thetag ID using the frequency that generates the greatest power value whichcorresponds to the closest tag reader. Put differently, whenbackscattering the received signals, the RFID tag selectively modulatesonly certain frequencies to include its ID. Doing so informs the tagreaders receiving the modulated signals whether it is the closet tagreader. In one example, if a tag reader is capable of transmitting andreceiving only 900 MHz signals but the RFID tag modulates thebackscattered signal at 200 MHz, then this tag reader cannot identify ordetect the RFID tag. In contrast, a tag reader that transmits andreceives signals at 200 MHz will detect the RFID tag, and by so doing,determine it is the closest tag reader to the tag. By selectivelymodulating the signal transmitted to the tag readers, the RFID tagconveys location information to one or more of the readers indicatingwhich reader is closest to the tag.

FIGS. 4A and 4B illustrate an RFID system for determining a location ofan RFID tag, according to one embodiment disclosed herein. The RFIDsystem includes a geographic area 400 that is defined into differentzones (Zone 1-4) that are respectively assigned to a tag reader 105. Inone embodiment, the zones represent regions in the geographic area 400where the strength of the signals transmitted by the respective tagreaders 105 is greater than the signal strength of the other three tagreaders 105. For example, when configuring the RFID system, a technicianmay walk the area 400 using a sensor to identify the zones (i.e.,regions) where the respective signal strengths are the highest. Althoughshown here as rectangles, this is a simplification of the zone since ina real world application the zones would likely have non-linearboundaries. Moreover, the technician may adjust the output power of thetag readers 105 so that the signal strength establish zones with thedesired boundaries—e.g., zones with equal sized areas or zones thatcover certain locations or features in the area 400.

The geographic area 400 may be an indoor facility such as a warehouse orstore, or an outdoor area such as a parking lot, amusement park, port,and the like. For example, the tag readers may be mounted to the roof ofa warehouse to monitor inventory using the RFID tags, or the tag readersmay be placed on poles in a parking lot to monitor cars in the lot.Moreover, the tag readers 105 do not need to be stationary but insteadcould move around in the area 400 which may cause the zones to adjustaccordingly. For example, a central controller 405 may detect that tagreader 105A has moved to change the boundary of Zone 1 and update theboundaries of all the zones accordingly.

In FIG. 4A, the tag 115 is located within Zone 3 which means the signaloutputted by tag reader 105C has the greatest strength. Even though tag115 is in Zone 3, the tag 115 may receive tag query commands (or othercommands) from tag readers 105 in other zones. For example, the tag 115may receive commands from tag readers 105A, 105C, and 105D, but not fromtag reader 105B which may be too far away. The tag readers 105 maytransmit RFID commands using four different frequency or frequencyranges. As such, the tag 115 may include four different receivers thatare each tuned to one of the frequencies used by the tag readers 105.Continuing the previous example, the three receivers in tag 115corresponding to the frequencies outputted by tag readers 105A, 105C,and 105D would detect RFID commands while the signal outputted by tagreader 105B may be too faint to be detected. As above, the receiverseach output a power value indicating the signal strength of thecorresponding frequencies. A comparator on the tag 115 determines whichsignal is the strongest at the location of the tag 115, which, in thiscase is tag reader 105C. As such, the tag 115 selectively modulates thesignal transmitted by the tag reader 105C to generate a reply messagethat includes the tag ID.

The tag readers 105 may be able to receive reply messages from the tag115 on more frequencies than the frequencies at which the readers 105transmit. In one embodiment, the tag readers 105 may be able to receivemessages at all the frequencies used by the other readers 105 totransmit commands to the tag 115. For example, tag readers 105A, 105C,and 105D may receive the reply message transmitted by the tag 115 at thefrequency outputted by tag reader 105C. However, because the modulationfrequency used by the tag 115 is different than the transmissionfrequency used by tag readers 105A and 105D, these readers 105 determinethat the tag 115 is not within their corresponding zones—i.e., zones 1and 4. This determination may occur using logic on the tag readers 105or may be decided by the central controller 405 which is coupled to allthe tag readers 105. For example, the tag readers 105 may transmitreceived responses to the central controller 405 which determines whichzone the tag 115 is located.

Moreover, the central controller 405 may use the fact that the tagreaders 105A and 105D receive reply messages from the tag, while tagreader 105B does not, to further narrow down the location of the tag115. Because the tag reader 105C receives the reply message from the tag115 at the frequency it transmits, the central controller 405 knows thetag 115 is in Zone 3. But because the tag readers 105A and 105D alsoreceive reply messages but tag reader 105B does not, the centralcontroller 405 can further narrow down the location of the tag 115 to asub-portion of the zone (referred to herein as quadrants of a zone). Putdifferently, the central controller 405 can use the knowledge thatneighboring tag readers 105 receive, or do not receive, the replymessage to divide each zone into quadrants or sub-portions.

In one embodiment, each zone may be divided into quadrants thatcorrespond to a particular combination of the tag readers 105 that do ordo not receive reply messages from the tag 115. For example, quadrant410 corresponds to when tag readers 105A and 105D receive the replymessage but tag reader 105B does not. However, if all three tag readers105A, 105B, and 105D received the reply message, then the correspondingquadrant may be the upper right hand corner of Zone 3. If only tagreader 105C receives the reply message, the corresponding quadrant maybe the lower left corner of Zone 3. Other quadrants in Zone 3 maycorrespond to other combinations such as if only tag reader 105A and105C receives the reply message or if only tag readers 105D and 105Creceive the message. In one embodiment, the different quadrants in thezones may overlap—e.g., the quadrant 410 corresponding to when tagreaders 105A and 105D receive the reply message may overlap with thequadrant corresponding to when only reader 105A receives the replymessage.

In another embodiment, instead of using quadrants to narrow down thelocation of the tag 115, the central controller 405 may use theinformation captured by the tag readers 105 to determine the location ofthe tag 115 within Zone 3. For example, the central controller 405 mayuse the knowledge of which tag reader 105 receives the reply message(and which do not) to perform triangulation or other spatial locationtechniques to determine the location of the tag 115.

Alternatively, each of the tag readers 105 may receive reply messagesonly at the frequency at which the reader 105 transmits. In thisembodiment, even if the reply message backscattered by the tag 115reaches tag readers 105A or 105D, these readers are unable to detect themessage. Thus, only tag reader 105C would receive the message therebyindicating to the central controller 405 that the tag 115 is in Zone 3.However, because the other readers 105 cannot detect the message, thecentral controller 405 may, if desired, rely on other locationtechniques to narrow down the location of the tag 115 in Zone 3.

In one embodiment, the tag readers 105 may use the knowledge of whichquadrant contains the RFID tag 115 to improve the accuracy of otherlocation detection techniques. As explained above, because the tag 115transmits a reply message using the transmission frequency of tag reader105C, this indicates to tag reader 105C that the tag 115 is located inZone 3. The tag reader 105C can also perform another location detectiontechnique such as measuring the received power of the reply message ormeasuring its time of flight of the reply message. These techniques mayidentify how far away the tag 115 is from the tag reader 105C in Zone 3,thereby further narrowing down the location of tag 115. Because the tagreader 105C already knows the tag 115 is in Zone 3, the reader 105C canignore multi-path signals which may be received from other zones in thegeographic area 400—i.e., Zones 1, 2, or 4. Ignoring multi-path signalsreceived from other zones may improve the accuracy of location detectiontechniques used when processing the reply messages transmitted by thetag 115.

In FIG. 4B, the tag 115 has moved into Zone 2 which corresponds to tagreader 105B. Stated differently, the receiver in tag 115 correspondingto the frequency transmitted by tag reader 105B outputs the highestpower value. In response, the tag 115 modulates the reply message at thesame frequency transmitted by the tag reader 105B, thereby indicating totag reader 105B (and the central controller 405) that the tag 115 isclosest to it. In this manner, the central controller 405 determinesthat tag 115 has moved from Zone 3 to Zone 2.

Like in FIG. 4A, the central controller 405 may also determine aparticular quadrant 415 in Zone 2 that includes the tag 115. Forexample, the reply message backscattered by the tag 115 may be receivedby tag readers 105A, 105C, and 105D in addition to reader 105B. Becausethis information is forwarded to the central controller 405, thecontroller 405 determines the tag 115 is within quadrant 415—i.e., thelower left corner of Zone 2.

In one embodiment, an interfering object may be between the closest tagreader (reader 105B in this case) and tag 115. This object may reflectsome or all of the signal emitted from the tag reader 105B to the tag115, thereby reducing the power value generated by the correspondingreceiver in the tag 115. As such, the tag 115 may erroneously determineits location is closer to a different tag reader. In one embodiment, thecentral controller 405 may time average the location informationcollected by the tag readers 105 before assigning the tag 115 to aparticular zone. For example, the interfering object (or the tag 115)may be moving in which case a drop in power value caused by the objectmay be only temporary. In one example, the interfering object may be aforklift navigating the geographic area 400 which temporary blocks muchof the signal transmitted by tag reader 105B from reaching tag 115. Thecentral controller 405 may evaluate multiple reply messagesbackscattered from the tag 115 before assigning the tag 115 to a zone.For example, while the forklift moves between tag 115 and tag reader105B, the backscattered reply messages may erroneously indicate the tag115 is in Zone 1. However, once the forklift is no longer interferingwith the signal, then the tag 115 may determine tag reader 105B is theclosest reader and generate reply messages correctly indicating that thetag 115 is in Zone 2. Thus, the central controller 405 may evaluatereply messages using, for example, a one to five minute interval whenassigning the tag 115 to a zone.

Moreover, when the tag 115 moves in the area 400, different locationsmay cause the signals transmitted by the tag readers 105 toconstructively or destructively interfere (i.e., cause dead spots).Thus, even if the tag 115 is in Zone 2, it may be located at a dead spotwhere the signals transmitted by tag reader 105B (which may includemulti-paths signals) destructively interfere at the current location ofthe tag 115. As such, time averaging the reply messages backscattered bythe tag 115 may enable the central controller 405 to ignore times whenthe tag 115 passes through a dead spot in Zone 2. Alternatively, tag 115may pass through a point in Zone 2 where the signals transmitted by tagreader 105D (which corresponds to Zone 4) constructively interfere,which causes the power value measured by the receiver in tag 115corresponding to reader 105D to jump. In some cases, the constructiveinterference may cause the power value corresponding to tag reader 105Dto be greater than the power value corresponding to tag reader 105B eventhough tag 115 is in Zone 2 and not Zone 4. Time averaging the relymessages sent by the tag 115 may also be helpful to identify locationsof constructive interference where signals from tag readers 105 areartificially increased within zones corresponding to different tagreaders 105.

In one embodiment, the RFID system may periodically shift the locationsof the tag readers 105 to different locations which changes the pointsof constructive and destructive interference in the zones. Doing sowhich may help in the situation where a tag 115 is stationary at pointof constructive or destructive interference in a zone. Even slightchanges in the location or orientation of the tag readers 105 can movethe locations of constructive and destructive interference in the area400. As such, the central controller 405 may change the locations ofconstructive and destructive interference without having to make largechanges in the current location of the tag readers 105, thereby keepingthe boundaries of the zones substantially unchanged.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

In the preceding, reference is made to embodiments presented in thisdisclosure. However, the scope of the present disclosure is not limitedto specific described embodiments. Instead, any combination of thefeatures and elements discussed above, whether related to differentembodiments or not, is contemplated to implement and practicecontemplated embodiments. Furthermore, although embodiments disclosedherein may achieve advantages over other possible solutions or over theprior art, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the scope of the present disclosure. Thus,the aspects, features, embodiments and advantages described herein aremerely illustrative and are not considered elements or limitations ofthe appended claims except where explicitly recited in a claim(s).Likewise, reference to “the invention” shall not be construed as ageneralization of any inventive subject matter disclosed herein andshall not be considered to be an element or limitation of the appendedclaims except where explicitly recited in a claim(s).

Aspects of the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.”

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1-7. (canceled)
 8. An RFID tag, comprising: a first receiver tuned toreceive a first RFID signal, wherein the first receiver is configured togenerate a first power value indicating a signal strength of the firstRFID signal; a second receiver tuned to receive a second RFID signal,wherein the second receiver is configured to generate a second powervalue indicating a signal strength of the second RFID signal, andwherein the first and second receivers are tuned to differentfrequencies; a comparator configured to determine which of the first andsecond RFID signals has a greater signal strength based on the first andsecond power values; and a transmitter configured to transmit a replymessage using a frequency corresponding to the first or second RFIDsignal with the greater signal strength.
 9. The RFID tag of claim 8,wherein the first RFID signal is a tag query command transmitted by afirst tag reader and the second RFID signal is a tag query commandtransmitted by a second tag reader.
 10. The RFID tag of claim 9, whereinthe first RFID signal comprises a frequency range that does not overlapwith a frequency range of the second RFID signal.
 11. The RFID tag ofclaim 8, further comprising: a modulator configured to modulate thefirst or second RFID signal with the greater signal strength to includean ID corresponding to the RFID tag.
 12. The RFID tag of claim 8,wherein the first and second receivers each comprises a power supplythat generates one of the respective power values.
 13. The RFID tag ofclaim 8, wherein the RFID tag is a passive tag, and wherein the powersupplies are configured to provide power to the transmitter fortransmitting the reply message.
 14. The RFID tag of claim 8, wherein thefirst receiver cannot detect the second RFID signal and the secondreceiver cannot detect the first RFID signal.
 15. An RFID system,comprising: a first tag reader configured to transmit a first RFIDsignal using a first frequency; a second tag reader configured totransmit a second RFID signal using a second frequency different fromthe first frequency; a RFID tag comprising: at least one receiverconfigured to receive the first RFID signal and the second RFID signal;and logic configured to: select, in response to receiving the first andsecond RFID signals, one of the first and second frequencies bydetermining which of the first or second RFID signal has the greatersignal strength, and output a reply message comprising a tag ID usingthe selected frequency.
 16. The RFID system of claim 15, wherein theselected frequency is the first frequency, and wherein the reply messageis not modulated using the second frequency.
 17. The RFID system ofclaim 15, wherein the first tag reader comprises a first receiver andthe second tag reader comprises a second receiver, wherein the firstreceiver cannot detect the reply message if the selected frequency isthe second frequency and the second receiver cannot detect the replymessage if the selected frequency is the first frequency.
 18. The RFIDsystem of claim 17, further comprising: a central controllercommunicatively coupled to the first and second tag readers, wherein thecentral controller is configured to: upon determining the selectedfrequency used to generate the reply message is the first frequency,assign a location of the RFID tag to a first zone corresponding to thefirst tag reader, and upon determining the selected frequency used togenerate the reply message is the second frequency, assign the locationof the RFID tag to a second zone corresponding to the second tag reader.19. The RFID system of claim 15, wherein the first tag reader comprisesa first receiver and the second tag reader comprises a second receiver,wherein both the first receiver and the second receiver can detect thereply message regardless whether the selected frequency is the firstfrequency or the second frequency.
 20. The RFID system of claim 19,further comprising: a central controller communicatively coupled to thefirst and second tag readers, wherein the central controller isconfigured to: upon determining (i) the selected frequency used togenerate the reply message is the first frequency and (ii) the secondtag reader received the reply message, assign a location of the RFID tagto a first quadrant in a first zone corresponding to the first tagreader, upon determining (i) the selected frequency used to generate thereply message is the first frequency and (ii) the second tag reader didnot receive the reply message, assign the location of the RFID tag to asecond quadrant in the first zone, upon determining (i) the selectedfrequency used to generate the reply message is the second frequency and(ii) the first tag reader received the reply message, assign thelocation of the RFID tag to a first quadrant in a second zonecorresponding to the second tag reader, and upon determining (i) theselected frequency used to generate the reply message is the secondfrequency and (ii) the first tag reader did not receive the replymessage, assign the location of the RFID tag to a second quadrant in thesecond zone.